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Results for “Atomic force microscopy (AFM)”

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PubMed1998

Measuring elasticity of biological materials by atomic force microscopy.

Atomic force microscopy (AFM) has proved its value not only for resolving the topographical structure of biological samples, but also for probing inherent properties of biological structures, like local interaction forces, mechanical properties or dynamics in a natural (physiological) environment. This minireview focuses on the acquisition of elasticity data of biomaterials by AFM. A possible theoretical model is presented, followed by a practical 'how to do it with AFM', and, finally, a brief overview of publications in this field is given.

Animals↗

PubMed2001

Characterisation of adhesional properties of lactose carriers using atomic force microscopy.

The atomic force microscopy (AFM) colloid probe technique was investigated as a method for the characterisation of adhesional properties of pharmaceutical powder surfaces. Lactose carriers used in dry powder inhaler (DPI) formulations were chosen for investigation since adhesion between the carrier surface and drug particles has been proposed to affect the dispersion of drug particles. Individual adhesion forces were determined by measuring the detachment forces in air between the colloid probe and the lactose particle surface. The colloid probe consisted of a silica sphere (10 microm diameter) attached to a V-shaped silicon nitride cantilever (spring constant, k=0.42 N/m). Adhesion forces were calculated from individual force-distance curves using Hooke's Law. Individual forces measured at various adhesion sites were observed to be reproducible and stable over 10 min (coefficient of variation, CV below 5%). The adhesion force distribution determined from measurements at multiple sites (n>50) on each sample followed a log-normal relationship (regression coefficient, r(2) ranged between 0.95 and 0.99). This enabled characterisation in terms of the geometric mean adhesion force and a geometric standard deviation (GSD). Significant differences (P<0.001) in adhesion force were observed between samples, ranging from 37.47+/-1.95 to 117.48+/-2.20 nN. This study demonstrates the suitability of AFM as sensitive technique for the characterisation of adhesional properties of pharmaceutical particles.

Adhesiveness↗

PubMed2006

Unbinding of the streptavidin-biotin complex by atomic force microscopy: a hybrid simulation study.

A hybrid molecular simulation technique, which combines molecular dynamics and continuum mechanics, was used to study the single-molecule unbinding force of a streptavidin-biotin complex. The hybrid method enables atomistic simulations of unbinding events at the millisecond time scale of atomic force microscopy (AFM) experiments. The logarithmic relationship between the unbinding force of the streptavidin-biotin complex and the loading rate (the product of cantilever spring constant and pulling velocity) in AFM experiments was confirmed by hybrid simulations. The unbinding forces, cantilever and tip positions, locations of energy barriers, and unbinding pathway were analyzed. Hybrid simulation results from this work not only interpret unbinding AFM experiments but also provide detailed molecular information not available in AFM experiments.

Biotin↗

PubMed2007

In-situ AFM studies of the phase-transition behavior of single thermoresponsive hydrogel particles.

The volume phase transition (VPT) behavior of individual thermally responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) hydrogel microparticles was studied by in-situ dynamic mode atomic force microscopy (AFM) and force spectroscopy during heating and cooling cycles. Hydrogel samples were prepared by electrostatic immobilization of microparticles to amine-modified gold surfaces. The AFM studies of particle deswelling were performed by varying the force applied on the particles during imaging as a function of the geometry and material of the AFM probe. Aluminum-coated silicon cantilevers were found to influence substantially the behavior of the particles during the VPT, leading to a significant shape change. Low force impact magnetic excitation of the AFM probe (MAC mode) during dynamic mode measurements resulted in an undisturbed deswelling behavior enabling observation of the expected volume changes of the particles without significant tip-sample interaction. Hence, MAC-mode AFM was determined to be the most suitable technique for in-situ AFM studies on volume and shape changes at single hydrogel particles during VPT. Elasticity measurements performed at single particles at temperatures below and above the VPT revealed a 15-fold increase in the Young's modulus after passing the VPT, indicating the transition from a soft, swollen network to a stiffer, deswollen state.

Acrylamides↗

PubMed2003

AFM study of potato virus X disassembly induced by movement protein.

Recently we have reported that a selective binding of potato virus X (PVX)-coded movement protein (termed TGBp1 MP) to one end of a polar coat protein (CP) helix converted viral RNA into a translatable form and induced a linear destabilization of the whole helical particle. Here, the native PVX virions, RNase-treated (PVX(RNA-DEG)) helical particles lacking intact RNA and their complexes with TGBp1 (TGBp1-PVX and TGBp1-PVX(RNA-DEG)), were examined by atomic force microscopy (AFM). When complexes of the TGBp1 MP with PVX were examined by means of AFM in liquid, no structural reorganization of PVX particles was observed. By contrast, the products of TGBp1-dependent PVX degradation termed "beads-on-string" were formed under conditions of AFM in air. The AFM images of PVX(RNA-DEG) were indistinguishable from images of native PVX particles; however, the TGBp1-dependent disassembly of the CP-helix was triggered when the TGBp1-PVX(RNA-DEG) complexes were examined by AFM, regardless of the conditions used (in air or in liquid). Our data supported the idea that binding of TGBp1 to one end of the PVX CP-helix induced linear destabilization of the whole helical particle, which may lead to its disassembly under conditions of AFM.

Capsid Proteins↗

PubMed2007

AFM fishing nanotechnology is the way to reverse the Avogadro number in proteomics.

Future development of proteomics may be hindered by limitations in the concentration sensitivity of widespread technological approaches. The concentration sensitivity limit (CSL) of currently used approaches, like 2-DE/LC separation coupled with MS detection, etc., varies from 10(-9) to 10(-12) M. Therefore, proteomic technologies enable detection of up to 20% of the protein species present in the plasma. New technologies, like atomic force microscopy (AFM molecular detector), enable the counting of single molecules, whereas biospecific fishing can be used to capture these molecules from the biomaterial. At the same time, fishing also has thermodynamic limitations due to the reversibility of the binding. In cases where the fishing becomes irreversible, its combination with an AFM detector enables the registration of single protein molecules, and that opens up a way to lower the CSL down to the reverse Avogadro number.

Microscopy, Atomic Force↗

PubMed2006

Visualization of single and aggregated hulless oat (Avena nuda L.) (1-->3),(1-->4)-beta-D-glucan molecules by atomic force microscopy and confocal scanning laser microscopy.

Surfactants were used to disperse oat beta-glucan. Atomic force microscopy (AFM) images of the resulting samples revealed a distribution of extended chainlike molecules and allowed, for the first time, direct visualization of single oat beta-glucan molecules with cross-sectional heights of about 0.44 nm. The number-average contour length (L(n)) and root-mean-square end-to-end distance ((R(ee)2)(1/2)) measured from the AFM images were 938 and 912 nm, respectively. The calculated persistence length (L(p)) was 526 nm. The weight-average molecular weight (M(w)) calculated from single beta-glucan molecules was 4.43 x 10(5). Samples without surfactant showed a strong tendency to form aggregates. The sample concentration, reserving time, and calcofluor as well as freezing could affect the formation of aggregates. These aggregates were visualized by both AFM and confocal scanning laser microscopy. The shape of the aggregates changed from small dots with diameters of approximately 20-50 nm to microfibrils over 3 microm long with the increasing of the concentration of oat beta-glucan from 10 to 100 microg/mL. The particle size distribution obtained by a laser particle size analyzer was 926 nm, which confirmed the size of oat beta-glucan molecules obtained from AFM images.

Avena↗

PubMed2005

Mechanically engraved mica surface using the atomic force microscope tip facilitates return to a specific sample location.

By controlling the interaction between the atomic force microscope tip and mica, patterns of different sizes and shape have been produced on the surface of mica. Using these operator-constructed patterns as a reliable marker, the original scanned sample location can be re-located and imaged again on the same mica surface by atomic force microscopy (AFM). This location technique can be used to find the same object again even if the sample was removed from the AFM instrument or the sample was imaged in a different mode.

Aluminum Silicates↗

PubMed2000

Surface modification using silanated poly(ethylene glycol)s.

Surface-grafted poly(ethylene glycol) (PEG) molecules are known to prevent protein adsorption to the surface. The protein-repulsive property of PEG molecules are maximized by covalent grafting. We have synthesized silanated monomethoxy-PEG (m-PEG) for covalent grafting of PEG to surfaces with oxide layers. Two different trialkoxysilylated PEGs were synthesized and characterized. The first trialkoxysilylated PEG was prepared by direct coupling of m-PEG with 3-isocyanatopropyltriethoxysilane through a urethane bond (silanated PEG I). The other silanated PEG (silanated PEG II) containing a long hydrophobic domain between PEG and a silane domain was prepared by reacting m-PEG with 1,6-diisocyanatohexane and 10-undecen-1-ol in sequence before silylation with 3-mercaptopropyl trimethoxysilane. Silanated PEGs I and II were grafted onto glass, a model surface used in our study. The PEG-grafted glass surfaces were characterized by contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Although contact angle did not change much as the bulk concentration of silanated PEG used for grafting increased from 0.1 to 20 mg/ml for both PEGs I and II, the surface atomic concentrations from XPS measurements showed successful PEG grafting. Surface PEG grafting increased concentration of surface carbon but decreased silicone concentration. The high resolution C1s spectra showed higher ether carbon with lower hydrocarbon compositions for the PEG-grafted surfaces compared to the control surface. AFM images showed that more PEG molecules were grafted onto the surface as the bulk concentration used for grafting was increased. AFM images of the dried surfaces showed that the surfaces were not completely covered by PEG molecules. After hydration, however, the surface appears to be covered completely probably due to the hydration of the grafted PEG chains. Glass surfaces modified with silanated PEGs reduced fibrinogen adsorption by more than 95% as compared with the control surface. Silanated PEGs provides a simple method for PEG grafting to the surface containing oxide layers.

Adsorption↗

PubMed2006

AFM studies of environmental effects on nanomechanical properties and cellular structure of human hair.

Characterization of cellular structure and physical and mechanical properties of hair are essential to develop better cosmetic products and advance biological and cosmetic science. Although the morphology of the cellular structure of human hair has been traditionally investigated using scanning electron microscopy and transmission electron microscopy, these techniques provide limited capability to in situ study of the physical and mechanical properties of human hair in various environments. Atomic force microscopy (AFM) overcomes these problems and can be used for characterization in ambient conditions without requiring specific sample preparations and surface treatment. In this study, film thickness, adhesive forces and effective Young's modulus of various hair surfaces were measured at different environments (humidity and temperature) using force calibration plot technique with an AFM. Torsional resonance mode phase contrast images were also taken in order to characterize the morphology and cellular structure changes of human hair at different humidity. The correlation between the nanomechanical properties and the cellular structure of hair is discussed.

Hair↗

PubMed2005

Combined AFM and confocal fluorescence microscope for applications in bio-nanotechnology.

We present a custom-designed atomic force fluorescence microscope (AFFM), which can perform simultaneous optical and topographic measurements with single molecule sensitivity throughout the whole visible to near-infrared spectral region. Integration of atomic force microscopy (AFM) and confocal fluorescence microscopy combines the high-resolution topographical imaging of AFM with the reliable (bio)-chemical identification capability of optical methods. The AFFM is equipped with a spectrograph enabling combined topographic and fluorescence spectral imaging, which significantly enhances discrimination of spectroscopically distinct objects. The modular design allows easy switching between different modes of operation such as tip-scanning, sample-scanning or mechanical manipulation, all of which are combined with synchronous optical detection. We demonstrate that coupling the AFM with the fluorescence microscope does not compromise its ability to image with a high spatial resolution. Examples of several modes of operation of the AFFM are shown using two-dimensional crystals and membranes containing light-harvesting complexes from the photosynthetic bacterium Rhodobacter sphaeroides.

Animals↗

PubMed2005

TAT peptide immobilization on gold surfaces: a comparison study with a thiolated peptide and alkylthiols using AFM, XPS, and FT-IRRAS.

A TAT peptide was used to functionalize a gold surface by three different methods: adsorption from solution, microcontact printing, and dip-pen nanolithography (DPN). The composition and structure of the modified gold was characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform -infrared reflection absorption spectroscopy (FT-IRRAS). We used two well-studied alkylthiols, mercaptohexadecanoic acid and 1-octadecanethiol, as a comparison in order to understand the structure of the TAT peptide monolayers prepared by the three methods. AFM studies allowed us to assess the homogeneity after each modification protocol. XPS was used to characterize the chemical composition of the gold surface after each functionalization procedure. The XPS results showed that surfaces modified with the TAT peptide by the three methods exhibit similar surface chemistry. Finally, FT-IRRAS experiments allowed us to conclude that the structure of the alkyl chains of the TAT peptides is fairly disordered and different after each procedure. Regardless of the type of surface functionalization method used, the monolayer of TAT peptide formed on the surface was of "liquidlike" nature.

Adsorption↗

PubMed2005

Polymer nanowire elastic moduli measured with digital pulsed force mode AFM.

The mechanical bending behavior of polymer nanowires-polypyrrole and poly(3,4-ethylene dioxythiophene-co-styrene sulfonate)-produced by template molding were measured using a new innovation in atomic force microscopy (AFM). Digital pulsed force mode (DPFM) was used to image and simultaneously perform three-point bend tests along nanowires spanning microchannels in silicon. The bending profiles were analyzed for apparent elastic moduli variations along the suspended length of individually isolated nanowires and compared to classic beam deflection models for various geometric and boundary conditions. The elastic moduli calculated from these AFM data are 2-7 times that expected for bulk polymer values (approximately 1-3 GPa), demonstrating an apparent strengthening of nanostructured polymer even for diameters greater than 100 nm--the accepted boundary for nanoscience. Furthermore, detailed analysis of deflection data versus loading location demonstrates the experimental dependence on test geometry and inherent errors in relying solely on midpoint bending measurements or any single loading configuration for nanomechanical testing as well as the significant contribution of nanoindentation effects.

Journal Article↗

PubMed2004

In vitro demineralization/remineralization cycles at human tooth enamel surfaces investigated by AFM and nanoindentation.

The early stages of enamel surface dissolution (erosion) are of high scientific relevance; however, little is known about if and to what extent these early stages, which result in the softening of the enamel surface, are reversible. The present study investigated the possible rehardening of surface softened enamel in two separate experiments in vitro, employing physiologically relevant demineralization and remineralization times and remineralizing solutions possessing chemical properties close to those of human saliva. Atomic force microscopy (AFM) based nanoindentation has been used to study the nanomechanical properties and the ultrastructure of polished enamel samples as affected by demineralization/remineralization cycles. In contradiction to previous studies employing microhardness techniques, no enamel rehardening has been observed after remineralization; however, the demineralization schemes used in the present study (short treatments with a citric acid buffer) are not comparable to the ones used in previous studies, making a comparison difficult. A mineral deposition after remineralization on softened enamel samples has been observed by AFM; however, a protective effect of this deposited layer could not be revealed. The exposure of softened enamel samples to either a remineralizing solution or a mineral water led to an improved acid resistance of these samples as shown in a demineralization/remineralization cycle. The present study has demonstrated that AFM based nanoindentation is a useful tool to investigate the demineralization and remineralization of surface softened enamel with high accuracy.

Dental Enamel↗

PubMed1999

Determining the molecular-packing arrangements on protein crystal faces by atomic force microscopy.

Previous atomic force microscopy (AFM) studies and periodic bond-chain (PBC) analyses of tetragonal lysozyme crystals have suggested that the (110) face consists of chains of molecules related to one another by 43 axes parallel to the crystal face. In this study, high-resolution AFM images of the (110) face were obtained and analyzed in order to verify this prediction. A computer program was employed which constructs the theoretical AFM image corresponding to a specific crystallographic molecular-packing arrangement and AFM tip shape. The packing arrangement and tip shape were varied in order to obtain the maximum possible correlation between experimental and theoretical images. The prediction from PBC analysis of an arrangement involving 43 helices was confirmed in this manner, while the alternate arrangement, consisting of molecules related to one another by 21 axes, was not observed. However, the surface structure was found to differ significantly even from this crystallographic arrangement. The molecules were found to pack slightly closer about what will become the 43 axes within the interior of the crystal, suggesting the occurrence of surface reconstruction or rearrangement on the tetragonal lysozyme (110) face. This study represents a new approach for more precise determination of the molecular-packing arrangements on protein crystal faces employing AFM.

Crystallization↗

PubMed2005

AFM studies of water-soluble wheat arabinoxylans--effects of esterase treatment.

The degradation products of water-soluble wheat arabinoxylans treated with Aspergillus niger ferulic acid esterase (FAEA-able to cleave 5,5'- and 8-O-4'-ferulic acid dimers) have been characterised by atomic force microscopy (AFM) and size exclusion chromatography. The AFM images of arabinoxylans confirmed that a small proportion ( approximately 15%) of the population of arabinoxylan molecules contain xylan-based branches attached to the xylan-based backbone. Treatment with FAEA reduced the contour length of the molecules suggesting that certain dimeric ferulic acid linkages may play a previously unconfirmed role in the elongation of arabinoxylans. Overnight treatment with FAEA led to a reduction in the density of branches suggesting that they may also be linked to the backbone through phenolic linkages.

Aspergillus niger↗

PubMed1997

AFM analysis of DNA-protamine complexes bound to mica.

A novel method for reconstituting sperm chromatin was used to investigate how protamine 1 condenses DNA. Complexes formed in vitro using linearized plasmid DNA were imaged and measured by atomic force microscopy (AFM). The structures formed were found to be highly dependent on the sample preparation method used for reconstitution. Interstrand, side-by-side fasiculation of DNA and toroidal-like structures only 1-2 DNA diameters thick were observed for complexes formed in solution following direct mixing of the DNA and protamine. Large chromatin aggregates were also observed on the mica. However, if the DNA was first allowed to attach to the mica prior to addition of the protamine, well-defined toroidal complexes were formed without any observed DNA fasiculation or aggregate formation. The diameter of the toroids measured 30.6-50.2 nm (mean 39.4 nm). The dimensions of these structures indicate that the condensed DNA is stacked vertically by four to five turns, with each coil containing as little as 360-370 bp of 'B'-form DNA. This approach for preparing and imaging DNA-protamine complexes permits the analysis of intermediate structures 'trapped' on the mica as partially formed toruses of nucleoprotamine.

Aluminum Silicates↗

PubMed2004

Evaluation of an atomic force microscopy pull-off method for measuring molecular weight and polydispersity of polymer brushes: effect of grafting density.

The accuracy of the molecular weights Mn and polydispersities of polymer brushes, determined by stretching the grafted chains using atomic force microscopy (AFM) and measuring the contour length distribution, was evaluated as a function of grafting density sigma. Poly(N,N-dimethylacrylamide) brushes were prepared by surface initiated atom transfer radical polymerization on latex particles with sigma ranging between 0.17 and 0.0059 chains/nm2 and constant Mn. The polymer, which could be cleaved from the grafting surface by hydrolysis and characterized by gel permeation chromatography (GPC), had a Mn of 30,600 and polydispersity (PDI) of 1.35. The Mn determined by the AFM technique for the higher density brushes agreed quite well with the GPC results but was significantly underestimated for the lower sigma. At high grafting density in good solvent, the extended structure of the brush increases the probability of forming segment-tip contacts located at the chain end. When the distance between chains approached twice the radius of gyration of the polymer, the transition from brush to mushroom structure presumably enabled the formation of a larger number of segment-tip contacts having separations smaller than the contour length, which explains the discrepancy between the two methods at low sigma. The PDI was typically higher than that obtained by GPC, suggesting that sampling of chains with above average contour length occurs at a frequency that is greater than their spatial distribution.

Journal Article↗